Didier Stevens

Tuesday 15 June 2021

Update: 1768.py Version 0.0.7

Filed under: My Software,Update — Didier Stevens @ 0:00

There are no code changes to this version of 1768.py, my tool to analyze Cobalt Strike beacons.

What is new, is file 1768.json: this file contains statistical data for license IDs.

Over a period of one month, I collected license ID information from these sources: threatviewio and @cobaltstrikebot.

For each license ID that is found on more than one IP address / hostname, I include simple statistics: the number of unique IP addresses / hostnames and the number of unique public keys.

When analyzing malicious Cobalt Strike beacons, I often see recurring license IDs. That’s why I decided to add logic and a JSON file to my tool, with license IDs I’ve seen before. And now this has evolved to a small repository of often seen license IDs.

Here is an example with a sample we discussed on the Internet Storm Center diary:

The license ID is 1873433027 and this ID is associated with 18 unique IP addresses / hostnames, and 15 unique public keys. This is a clear indication that this license ID is used by malicious actors. License IDs that have been seen only once, could belong to red teams, that is why they are not included in file 1768.json. The more often a license ID is seen, the higher the chance it is used by malicious actors. Of course, it is not excluded that there are legitimate license IDs from red teams in this list, but I expect they will have low frequencies.

Takeaway: if your sample has a license ID that appears in 1768.json, then it has been seen before (at least twice), and you’re likely not dealing with a pentest.

1768_v0_0_7.zip (https)
MD5: D93AC5707FD0B5315A1225121071C7F2
SHA256: B417790451681643B2269AC516A99F3CEE9F7F374AB529FD53D5702A70F79409

Friday 11 June 2021

New Tool: ssdeep.py

Filed under: My Software — Didier Stevens @ 10:35

ssdeep.py is a Python tool to calculate ssdeep hashes using the ppdeep Python module.

As I needed a Python implementation of an ssdeep tool, I decided to document the creation of such a tool with a video. I use my Python templates to quickly create this tool.

ssdeep_V0_0_1.zip (https)
MD5: 32FD610D858E91BC009845E105ED87C3
SHA256: 02EA18EF0139B54D8A06AA0D3E7E2B0E2934E3675C453759E3DA3CC4F936F0A2

Update: Python Templates Version 0.0.5

Filed under: My Software,Update — Didier Stevens @ 10:14

Here is an update to my Python templates.

I use these templates as a starting point for new tools or for quick development of ad-hoc tools.

I also recorded a video showing how to use my template to create your own tool: ssdeep Python Example Based On My Templates.

python-templates_V0_0_5.zip (https)
MD5: 137878F4D7F799436F76C0119E6BB621
SHA256: 5A68B115B5616BC35CFB4DDEA64C029BF10DDCD6BFF5E4B9D3D4DBBC0FBD6651

Sunday 30 May 2021

New Tool: cs-dns-stager.py

Filed under: My Software — Didier Stevens @ 17:59

cs-dns-stager.py is a quick & dirty tool I wrote to retrieve a Cobalt Strike DNS beacon from its server, if you only have the IP address of said server.

If you want to know more about Cobalt Strike and DNS, watch this video I recorded:

Tuesday 25 May 2021

Update: base64dump.py Version 0.0.14

Filed under: My Software,Update — Didier Stevens @ 0:00

This new version of base64dump.py supports a new encoding: NETBIOS Name encoding.

NETBIOS Name encoding is very similar to hexadecimal encoding: in stead of hexadecimal digits 0-9 and a-f, letters A-P are used.

I encountered this in DNS TXT records of a Cobalt Strike DNS stager. More on that later.

base64dump_V0_0_14.zip (https)
MD5: 35BF4900BED40E828887C7601F9C8751
SHA256: 2F58F630D9B12D2B70CECF35728096A247890808E44DAB9C94400A073D5E29BF

Sunday 23 May 2021

Update: re-search.py Version 0.0.17

Filed under: My Software,Update — Didier Stevens @ 0:00

This new version of re-search.py adds gzip support and filtering of private IPv4 addresses:

re-search_V0_0_17.zip (https)
MD5: 8945F435BDA03D73EF7A2BA1AA64A65E
SHA256: 0D74709B9F26FC7F6EEADAEE1BAA3AF7AADAA618F88B1C267BA5A063C8E3D997

Saturday 22 May 2021

Update: 1768.py Version 0.0.6

Filed under: My Software,Update — Didier Stevens @ 15:06

This new version of 1768.py, my tool to analyze Cobalt Stike beacons, has fixes, support for more encodings, and an option to output the config in JSON format.

1768_v0_0_6.zip (https)
MD5: EB9C949BB7B5DD3EF9ECEBF7F3C21184
SHA256: 3EC0BB7B41CC5C0E1534F09BAE67D62B220F8D83A7F02EC0F856F8741F86EB31

Monday 26 April 2021

Quickpost: Decrypting Cobalt Strike Traffic

Filed under: Encryption,Malware,My Software,Quickpost — Didier Stevens @ 0:00

I have been looking at several samples of Cobalt Strike beacons used in malware attacks. Although work is still ongoing, I already want to share my findings.

Cobalt Strike beacons communicating over HTTP encrypt their data with AES (unless a trial version is used). I found code to decrypt/encrypt such data in the PyBeacon and Geacon Github repositories.

This code works if you know the AES key: which is not a problem in the use cases of the code above, as it is developed to simulate a beacon. Beacons generate their own AES key, and thus these beacon simulations also generate their own AES key.

But what if you’re analyzing real beacons used in malware attacks? How do you obtain the AES key?

I found a way to extract the keys (AES and HMAC) from process memory of a running beacon.

I use the following procdump command to prepare process memory dumps:

procdump -mp -w -s 1 -n 5 malware.exe

Then I start the beacon malware.exe in a malware analysis virtual machine while capturing traffic with Wireshark.

My new tool cs-extract-key.py looks in the dumped process memory for the unencrypted (RSA encryption) metadata that a beacon sends to the C2. This metadata contains the AES en HMAC keys.

Example:

This method does not always work: the metadata is overwritten after some time, so the process dump needs to be taken quickly after the beacon is started. And there are also cases were this metadata can not be found (I suspect this is version bound).

For those cases, my tool has another way of obtaining the keys. I extract the encrypted data of the first post of the beacon to the C2 (this is called a callback in the PyBeacon code):

And then I provide this to my tool, together with the process dump. My tool will then proceed with a dictionary attack: extract all possible AES and HMAC keys from the process dump, and try do authenticate and decrypt the callback. If this works, the keys have been found:

And once I have obtained the keys, I can pass them to my traffic decoding program that I have updated to include decryption (and that I have renamed to cs-parse-http-traffic.py):


Quickpost info


Sunday 25 April 2021

isodump.py

Filed under: My Software,video — Didier Stevens @ 10:13

This is a new tool (beta) to analyze ISO files. I made this for a webinar I presented: a demo on how to use my templates to create your own tools.

isodump.py is in my Github beta repository.

The complete webinar is here, if you want to jump directly to the demo where I explain how to make a tool like isodump.py, go here.

Monday 19 April 2021

Lua CSV Wireshark Dissector

Filed under: My Software,Networking,Wireshark — Didier Stevens @ 0:00

In December 2020 I provided online Wireshark training to one of our NVISO clients. During the second day, when we cover the development of custom dissectors written in Lua, a question about CSV data came up. When the data exchanged over TCP, for example, has the CSV format (fields separated by a separator), how can I write a dissector for that?

While answering the question, I realized that this is a case that could be solved with a generic dissector. And the same night, I developed the first version.

Say you have a packet capture with a TCP connection. And the data exchanged over TCP consists of different fields, separated by a separator character.

Like this example:

Because Wireshark does not recognize the protocol used in this TCP connection, the content is just displayed as data.

With Lua dissector csv-dissector.lua, the data is dissected into different fields:

The separator character (pipe character | in this example) is something that can be configured:

Other changes can be made, but these have to be made in the code of the dissector itself:

  • Changing the port
  • Changing the number of fields
  • Change the name of the fields

 

Download:

csv_dissector_V0_0_2.zip (https)

MD5: E8CCE089FB0574775AB39DADED3B7AA2

SHA256: 5C8DC0F2BB97AA660E2576B23379B6F12FB88126F0EFC7A2F69E76EBA8E782BD

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